This invention relates to a solid electrolytic capacitor using a polymer as an electrolyte and, more specifically, relates to a thin-type surface-mount capacitor.
As capacitors of this type, there have been known those capacitors described in, for example, Japanese Patent Application Publication (JP-A) H05-275290 (hereinafter, the capacitor described in this publication will be referred to as the solid electrolytic capacitor according to the prior art 1) and Japanese Patent Application Publication (JP-A) 2002-313676 (hereinafter, the capacitor described in this publication will be referred to as the solid electrolytic capacitor according to the prior art 2).
The solid electrolytic capacitor according to the prior art 1 is called the two-terminal mold type. In this capacitor, a conductive functional polymer film is used as a solid electrolyte. The conductive functional polymer film is formed on an anode oxide film of a valve-action metal anode body so as to cover one end portion of the anode body, and a conductive layer is further formed around the conductive functional polymer film, thereby to form a cathode layer. Leads are connected to the anode body and the cathode layer and drawn therefrom to a lower side to expose their end portions. Thereafter, the composite of the foregoing components is overmolded with a casing resin so that a surface-mount type capacitor is formed.
The solid electrolytic capacitor according to the prior art 2 is called the three-terminal transmission line element type that is directly connected to a power circuit so that the current flows inside. In this capacitor, like the prior art 1, a conductive functional polymer film is used as a solid electrolyte. The conductive functional polymer film is formed on an anode oxide film of a valve-action metal anode body so as to cover a central portion thereof, and a conductive layer is further formed around the conductive functional polymer film, thereby to form a cathode layer. Anode terminals are respectively joined to both ends of the anode body, and the cathode layer is covered with a thermal adhesive resin impregnated tape having a through hole. After filling the through hole with a conductive paste, a cathode terminal metal plate covers the tape including the conductive paste so as to form a cathode terminal. In this manner, a surface-mount type capacitor is formed.
In the two-terminal mold type solid electrolytic capacitor according to the prior art 1, invasion of oxygen is prevented by molding of the casing resin. However, by means of the molding, it is difficult to increase adhesion between an anode terminal and the casing resin for tightly sealing therebetween. Oxygen in the atmosphere may enter the inside of the capacitor from a joining portion between the anode terminal and the casing resin to thereby oxidize the conductive functional polymer film. Therefore, the preventive measure against the invasion of oxygen is not sufficient. Further, the structure of this capacitor also has a drawback that the size increases.
For solving such a problem, it may be considered to apply gold plating to the terminal or inject a sealing agent. However, this causes an increase in production cost.
On the other hand, in the three-terminal transmission line element type solid electrolytic capacitor according to the prior art 2, there is a large gap between an element reinforcing metal plate and the cathode terminal metal plate, and further, the area of the thermal adhesive resin impregnated tape on the cathode terminal side is equal to or smaller than the area of the cathode layer. Therefore, the thermal adhesive resin impregnated tape does not fully cover the cathode layer including the conductive functional polymer film, and thus there are exposed portions to easily allow invasion of oxygen. Accordingly, like the prior art 1, the prior art 2 also has a drawback that the electrolyte layer in the form of the conductive functional polymer film is liable to be oxidized.